A known track-type work machine has crawler belts each of which is wrapped around a driving sprocket rotatively driven by a power from a driving source supported by a main frame and around an idler movably supported by the main frame. The crawler belts are each guidingly supported by track rollers at the ground engaging side and by track carrier rollers at the ground unengaging side. For creating tension in the crawler belts, a track tensioning mechanism of the coil spring type or hydraulic cylinder type is interposed between a yoke for supporting a bearing for the idler and a track frame. The crawler belts are constructed by coupling a plurality of track links and therefore, slack is created because the track link components (e.g., links and bushings) and idlers begin to wear over a period of time. Therefore, there arises a need to increase tension on the crawler belts in order to eliminate slack in the crawler belts.
A means for increasing the tension of crawler belts is disclosed in Japanese Patent Kokai Publication No. 7-144668. According to the track adjuster disclosed in Japanese Patent Kokai Publication No. 7-144668, grease is manually injected into a grease chamber of a grease cylinder to adjust the tension of the crawler belts.
Another track adjuster is disclosed in Japanese Patent Kokai Publication No. 2000-247273. According to this publication, the track adjuster including hydraulic cylinders (adjuster cylinders) is used for adjusting tension on a crawler belt provided on both sides of a work machine. This track adjuster is provided with an adjuster cylinder control circuit which is operated so as to properly absorb stroke fluctuations when a load is imposed on both of the hydraulic cylinders and when a load is imposed on either of the hydraulic cylinders.
An arrangement for optimizing adjustment of the tension of crawler belts by hydraulic cylinders is disclosed in Japanese Patent Publication No. 2001-206261.
The track adjuster of Japanese Patent Kokai Publication No. 7-144668 has revealed the following problems. (1) Since the operator makes an adjustment using grease, it is impossible to fit the tension of the crawler belts to both situations, that is, the situation where the work machine is in a forward movement and therefore the crawler belts may be relatively loosened and the situation where the work machine is in a backward movement and therefore the crawler belts should be tightly stretched. The use of this track adjuster causes a decrease in the efficiency associated with operation of the work machine. (2) If a crawler belt bites foreign materials such as rocks during vehicle traveling or if earth and sand penetrate into and deposit on the tooth roots of a driving sprocket, causing abnormal tension on the crawler belt, the load on the coil spring increases, imposing a heavy load on every parts of the track frame. In view of this, the work machine needs a sturdy structure, which leads to an increase in vehicle weight and therefore additional cost. (3) When the aforesaid biting state is cleared, the accumulated energy of the coil spring is released all at once. The components of the crawler belts are, therefore, required to have enough strength to withstand the impulsive load occurring at that time, which leads to an increase in the weight of the components and therefore additional cost. (4) Since the size of the coil spring is dependent of the material used, miniaturization of the coils spring is difficult. Therefore, the track frame which houses the coil spring is required to be large in size. This hampers an improvement in the soil removing capability of the work machine.
Track adjusters utilizing hydraulic cylinders such as disclosed in Japanese Patent Kokai Publication No. 2000-247273 are operated by an incompressible fluid unlike the coil spring type tensioning mechanisms and therefore require an accumulator. For ensuring the reliability of the piping system, the accumulator needs to be periodically charged with gas and periodical piping (hydraulic hoses) replacement becomes necessary. In addition, it is necessary to run piping from the hydraulic driving source to the undercarriage assembly in which the track adjuster is incorporated, and the space for accommodating the hydraulic piping (mainly hydraulic hoses) is needed, which is accompanied with many problems in manufacturing.
The track adjuster utilizing hydraulic cylinders according to Japanese Patent Kokai Publication No. 2001-206261 is designed to have a positional sensor used in control performed in synchronization with the expansion/contraction of the crawler belts. Such a control system is seemingly rational, but imposes the following problem: Since work machines such as bulldozers are usually operated on the unleveled ground and therefore contaminated with earth and sand at their crawler units, the light beam of the sensor is likely to be cut off by earth and sand during operation in cases where a general photoelectric sensor is used as a high-precision positional sensor. Therefore, such a sensor lacks accuracy. Even when a bobbin-shaped scale is used as a means for measuring distance in addition to the laser, if earth and sand penetrate into the measuring part during measurement (operation), the measuring function will be hampered, so that intended aims cannot be accomplished. Further, in this track adjuster, the right and left crawler belts need to be separately adjusted. Therefore, running of hydraulic conduits from the vehicle body to the undercarriage for the purpose of piping from the hydraulic driving system (i.e., the driving source for the track unit and other instruments disposed on the main frame for performing control) to the hydraulic cylinders gives rise to difficulties in reservation of a route, as pointed out earlier. This causes many problems not only in manufacturing but also in maintenance.
The invention is directed to overcoming the foregoing shortcomings and a primary object of the invention is therefore to provide a crawler track tension adjusting device in which a control operation mechanism is operated by a driving unit of a closed structure and the tension of crawler belts is automatically adjusted in response to a control signal to optimize it.